This invention relates generally to illumination detection and more specifically to a non-imaging detector for directly measuring the incidence angle of an illuminating source from a plane normal to the detector.
The detector and its relatively simple electronic circuitry will generate an output signal having an amplitude that is proportional to the angle that an illuminating source, such as an aircraft landing light or distant aircraft strobe light, is displaced from one plane normal to the surface of the detector. For example, the detector may be positioned and aligned to detect an azimuth angle from a particular reference, such as an airport runway heading, and when subjected to irradiation from an aircraft landing light, will produce an output signal that is proportional to the azimuth angle of the aircraft from the runway heading. If the measurement of a horizontal angle of approach is required, a second detector system appropriately aligned to detect a flight path or elevation angle must be employed. The detectors are designed so that they are completely insensitive to illumination variations in the orthogonal axis and there is no cross interference between detectors operating in orthogonal planes.
The detector to be described is insensitive to illumination bursts or variations and will continue to provide accurate and non-varying angle indicating output signals whenever the approaching aircraft is maneuvering and the light beam is turned toward and from the detector. Thus, two or more detectors operating with their associated electronic circuits may be employed in a system for directing nighttime aircraft landings or for other applications where the direction of an illuminating source is required.
The detector and associated circuitry may conveniently be subminiaturized and can be manufactured in quantity at relatively low cost by conventional deposition techniques employed in the production of electronic integrated circuitry. The detector array is formed of a relatively large plurality of thin adjacent parallel strips of detector material having a suitable sensitivity to the illumination to be detected. Overlying each of the parallel strips is a mask deposited to reveal open areas comprising a plurality of identical detector elements in series and of a particular configuration on the surface of the detector material. Running longitudinally through each series detector in a detector strip is a deposited electrical conductor for transmitting the photodetector signal to associated external circuitry. The mask pattern is reversed in each adjacent strip and the electrical conductors connected to the detectors of each of these alternate strips of the array are interconnected so that the photocurrents flow in opposite directions in the conductors in alternate strips.
Closely overlying the array of detectors but spaced therefrom is an upper opaque mask having a plurality of narrow parallel slits or transparent bars that are aligned laterally to the longitudinal axis of each of the parallel detector strips. The equal spacing between each slit in the plurality corresponds with the length of each detector element in each parallel strip developments and the center line of each slit is precisely positioned over, and parallel with, a lateral line across the center of each element. Thus, illumination from the source precisely normal to the surface of the top mask will be projected as narrow lines of illumination across the detector array with each projected line crossing the center of the detectors in the adjacent parallel strips.
As mentioned above, the plurality of series connected detectors in adjacent parallel strips have reversed mask patterns so that their respective output currents flow according to the pattern directions. When the projected illumination through the slits of the upper mask fall across the centers of the detectors in adjacent arrays, each detector is equally excited and the photocurrents from adjacent strips are equal so that their difference is therefore equal to zero. But the masks in alternate detector strips are shaped so that there is progressively less irradiation, hence lower photocurrents, as the illumination through the top mask moves in a longitudinal direction from the center of the detector cells. Therefore, as the angle of an illuminating source changes, the detectors in one alternate strip will generate less currents while the other alternate strips of detectors may continue to generate the same, or greater currents depending upon the detector mask design. Thus, as the illumination angle changes, the difference in photocurrents from detectors in alternate strips will increase. It would appear to be a simple task to determine the angle to the illuminating source by measuring the total currents from the alternate strips in the array and performing the operation. Unfortunately the problems associated with performing this division while maintaining a desired sensitivity and dynamic range are formidable and costly and often results in errors that render the approach impractical.